Ordinary internal combustion engines for vehicles include multiple-cylinder internal combustion engines having multiple cylinders. In multiple-cylinder internal combustion engines, cylinders are divided into a plurality of groups, and an exhaust emission purifying device is installed at each of the plurality of cylinder groups in some cases. For example, in the V-type engine described in Japanese Patent Laid-Open No. 2008-038866, the exhaust emission purifying device can be installed at each of the left and the right banks. In such a case, from the viewpoint of emission performance, the air-fuel ratio is desirably controlled for each of the cylinder groups. That is to say, it is desirable to set a request air-fuel ratio for each of the cylinder groups and control fuel injection amounts of the respective cylinders in accordance with the request air-fuel ratios to the respective cylinder groups.
An important control variable of an internal combustion engine as well as the air-fuel ratio described above is the torque generated by the internal combustion engine. For example, Japanese Patent Laid-Open No. 2010-007489 and Japanese Patent Laid-Open No. 2010-053826 each disclose the method that acquires the request torque and the request air-fuel ratio to the internal combustion engine, and determines the respective control variables of the throttle, the ignition device, and the fuel injection device, in order to realize the request torque and the request air-fuel ratio. Concerning the throttle, a throttle opening which is the operation variable thereof is determined in accordance with the target air quantity for realizing the request torque. By using, for example, the inverse model of an air model, the throttle opening which is necessary for realization of the target air quantity can be obtained by calculation.
Incidentally, an air-fuel ratio is also closely related to the torque which an internal combustion engine generates, in addition to the quantity of the air taken into the cylinder. When the air quantities are the same, if the air-fuel ratio of the air-fuel mixture which is provided for combustion is leaner than stoichiometry, the torque decreases, and if the air-fuel ratio of the air-fuel mixture which is provided for combustion is richer, the torque increases. Therefore, in the process of converting the request torque into a target air quantity, the air-fuel ratio, that is, the request air-fuel ratio, of the air-fuel mixture in the cylinders is desirably referred to. By setting the target air quantity in response to the request air-fuel ratio, achievability of the request torque can be enhanced.
However, in the above case, the problem as follows arises.
Even in the case of the internal combustion engine which has a plurality of cylinder groups like a V-type engine, one throttle is used in general, and the throttle is shared by a plurality of cylinder groups. Therefore, the target air quantities for a plurality of cylinder groups have to be realized by operation of the one throttle. When the request air-fuel ratios differ between the cylinder groups, the target air quantities which are determined with reference to the request air-fuel ratios differ between the cylinder groups. If the target air quantities differ between the cylinder groups, the target throttle openings which are required for realization thereof are also set at different values. As a result, the target throttle opening which is used in operation of the throttle varies from one cylinder to another during one cycle.
In FIG. 6, a state thereof is shown in charts. A chart in an uppermost stage shows a change with time of request torque. A second chart shows a change with time of a request air-fuel ratio for a right bank. A third chart shows a change with time of a request air-fuel ratio for a left bank. A chart in a lowermost stage shows a change with time of a target throttle opening. In the case shown in FIG. 6, the request air-fuel ratios are made rich both at the left and the right banks as an initial state, and at a time point t1, only the request air-fuel ratio of the right bank is returned to stoichiometry. Thereafter, at a time point t2, the request air-fuel ratio of the left bank is also returned to stoichiometry. That is to say, in the case shown in FIG. 6, during a time period from the time point t1 to the time point t2, the request air-fuel ratios differ between the left and the right banks.
In the engine having left and right banks, the request air-fuel ratios of the left and the right banks are alternately read in accordance with the ignition sequence when the target throttle opening is calculated from the target air quantity. The target air quantity is determined with reference to the request air-fuel ratio, and therefore, during the time period from the time point t1 to the time point t2, the target air quantity oscillatorily changes in response to the request air-fuel ratios which are read. As a result, the target throttle opening which is converted from the target air quantity also shows an oscillatory change as shown in the chart on the lowermost stage of FIG. 6.
When the target throttle opening varies as shown in FIG. 6, the throttle is oscillatorily moved at a high frequency. However, a change of the air quantity with respect to the operation of the throttle has a response delay, and therefore, the oscillatory operation of the throttle may make the air quantity unstable. As a result, not only the achievability of the request torque and the request air-fuel ratio declines due excess or deficiency of the air quantity, but also extreme excess or deficiency of the air quantity could cause worsening of combustion. Further, there is the fear that the request air-fuel ratios to the respective banks on the left and the right cannot be realized with sufficiently high precision.